Magnetic domain microstructure analysis of sintered rare earth magnet materials with high coercivity

Abstract

Rare earth magnet materials are commonly used in many applications, such as the motors in hybrid and electric vehicles. To improve the magnetic performance of these materials at high temperatures, certain heavy rare earth elements are added to the structure. The high demand and scarcity of heavy rare earth elements have led to a considerable increase in rare earth prices. In this context, more research is being conducted to develop of free rare earth magnets that combine high coercivity and high thermal resistance. The coercivity of sintered permanent magnet materials is controlled by the microstructural parameters of magnet such as grain shape, grain size, grain boundary, and phase morphology. As part of this research effort, a great understanding of the microstructural analysis of materials is vital. In this thesis, different areas have been identified in which improvements can be made in understanding the magnetic domain microstructure of sintered Nd-Fe-B and SmCo magnets with high coercivity. Firstly, this thesis describes the use of magnetic force microscopy (MFM) to investigate the domain structure of sintered rare earth magnet materials in a thermally demagnetised state. Observations are made at both perpendicular and parallel surfaces to the alignment axis. Maze-like magnetic domains and stripe magnetic domains are observed in the perpendicular and parallel surfaces respectively. Branched-like domain patterns are imaged for sintered Nd-Fe-B and Sm2Co17 magnets in the parallel alignment axis. Clear magnetic contrast images in 3D view with defined domain structures are presented. An important microstructural domain patterns are shown in the 3D MFM images such as spike domains and reverse spikes. Samples with wider magnetic domains have spike domain regions, whereas samples with narrower domains exhibit reverse spikes. Secondly, a model is developed that determines the microstructural parameters, such as domain width and domain wall energy, for sintered Nd-Fe-B and SmCo magnets, from the observed MFM images. The domain width and domain wall energy have been determined in the parallel and perpendicular alignment directions. A surface parallel to the alignment axis exhibits a higher domain width and energy compared to that in the perpendicular direction. It is shown that just changing the annealing temperature can have a major effect on the domain width and domain wall energy that leads to change the magnetic properties. In addition the ratio of root-mean-square values for MFM phase images is found to be a good indicator for the variation of magnetic properties. Finally some distorted regions on the topographic image are observed which have a clear effect on MFM image. The effect of the distorted regions on the domain walls is studied using MFM. These regions have been identified using Raman spectroscopy. In addition, the impact of increasing temperature on the distorted regions and their corresponding domain walls is studied using MFM and Raman spectroscopy. The observation are made in-situ and at different locations of sample surface. The distorted regions are increased as the temperature raise. New methods are presented to compute the size of distorted domain walls. The techniques are 2D line profile method and watershed method. The samples studied include both sintered Nd-Fe-B and SmCo materials aimed at understanding the microstructure of sintered Nd-Fe-B and SmCo magnets with a high coercivity.